1. Field of the Invention
The invention relates to an active-drive type pixel structure comprising at least TFTs (Thin Film Transistors) for control and drive, and a capacitor for charge retention, and to an inspection method therefor. More particularly, the invention relates to an active-drive type pixel structure and to an inspection method therefor, in which it is possible easily to inspect, before a pixel is formed, for example, a light emitting element is deposited, whether the functions of the above-described TFTs and that of the capacitor for charge retention are normal or not.
2. Description of the Related Art
A display using a display panel comprising light emitting elements arranged in a matrix has been widely developed. Organic EL (electro-luminescence) element using an organic material for a light emitting layer has been noticed as a light emitting element used for such a display panel. The reason is that the element has had very great practical utility, high efficiency, and a long life by using, for the light emitting layer of the EL element, an organic compound by which excellent light emitting characteristics can be expected.
There has been proposed, as a display panel which uses the organic EL element, a simple matrix type display panel in which the EL elements are simply arranged in a matrix and an active matrix type display panel in which each of the EL elements arranged in a matrix is provided with active elements comprising TFT. The latter active matrix type display panel can advantageously realize less electricity consumption and more reduced cross talk between pixels in comparison with those of the former simple matrix type display panel, and, especially, is suitable for a high-definition display forming a large screen.
FIG. 1 shows a most basic circuit structure for one pixel 10 in a conventional active matrix type display and the structure has been called as a conductance control method. In FIG. 1, a gate G of TFT (Tr1) for control, which comprises n-channels, is connected to a scanning line 1a from a scanning driver 1 and its source S is connected to a data line 2a from a data driver 2. And, a drain D of the TFT (tr1) for control is connected to the gate G of TFT (Tr2) for drive, which comprises P-channels, and also to one terminal of a capacitor C1 for charge retention.
Moreover, a source S of the TFT (Tr2) for drive is connected to the other terminal of the above-described capacitor C1, and also to a power supply at the side of an anode (VHanod) which supplies a driving electric current to an organic EL element E1 as a light emitting element. Furthermore, a drain D of the TFT (tr2) for drive is connected to an anode of the above-described organic EL element E1, and a cathode of the EL element in question is connected to a power supply at the side of a cathode (VLcath).
The TFT (Tr1) for control passes, from the source to the drain, an electric current corresponding to a data voltage (V data) which is supplied from the data line 2a to the source, when an ON-state control voltage (Select) is supplied to the gate of the TFT (Tr1) for control in FIG. 1 through the scanning line 1a. Accordingly, the above-described capacitor C1 is charged during the ON-state voltage at the gate of the TFT (Tr1) for control, and the voltage is supplied to the gate of the TFT (Tr2) for drive. Then, the TFT (Tr2) for drive passes the electric current based on the gate voltage and the source voltage to the EL element E1 and the element E1 is driven into light emitting.
Moreover, though the TFT (Tr1) for control is put into a so-called CUT-OFF state when the voltage of the gate of the TFT (Tr1) for control becomes an OFF-state voltage and the drain of the TFT (Tr1) for control is put into an open state, the gate voltage of the TFT (Tr2) for drive is maintained by charges accumulated in the capacitor C1, the driving electric current is maintained, and light emitting of the EL element E1 is maintained till the next scanning.
The above-described configuration shows one connection configuration example of the pixel 10 by the conductance control method, in which an image is reproduced by arranging a number of the pixels 10 in the vertical and horizontal directions and controlling each pixel for turning on or off, based on an image signal.
Incidentally, defects of TFT and a capacitor in each pixel cause a defect in pixels in this kind of the active matrix type display panel. Though it is unavoidable in the present situation to cause some defects in the display panel, the quality of the display panel is deteriorated to make the panel unsuitable as a commodity when the number of defects increases.
Therefore, if it is possible easily to inspect the above-described TFTs and capacity for charge retention for defects in the state of a semi-processed product before a state in which the above-described TFTs and capacitor for charge retention are deposited on a substrate, that is, a state in which an organic EL element as a light emitting element is formed on the above-described substrate, it is possible to improve a yield rate of the display panel. As a result, it is possible to contribute to the cost reduction. Especially, in comparison with the case of AM-LCD (active matrix type liquid crystal display) in which only one TFT is required for each pixel, inspection for defects in the above-described state of a semi-processed product becomes more important in AM-OEL (active matrix type organic EL display) in which equal to or more than two through four pieces of TFTs are required for each pixel.
On the other hand, since the capacitor for charge retention is a load of TFT for a pixel (TFT for drive) even in the above-described state of a semi-processed product, that is, in the state of the TFT substrate, it is comparatively easy in AM-LCD to execute inspection for defects even in the state of a TFT substrate. However, the TFT for drive is in a no-load state in the case of AM-OEL because the organic EL element is not deposited on the TFT substrate in the above-described state of a semi-processed product. Accordingly, it is not easy in such a state to execute inspection for defects of pixels.
Accordingly, a method, in which a probe is contacted to a predetermined picture element electrode and the like to measure impedance for inspection of defects of pixels, has been proposed in Japanese Patent Publication NO. 2506840 (after the 15th line in the second column and FIG. 6). Therefore, there is considered a similar method in which, in order to inspect pixels for defects, a load is connected to the TFT for drive for example, by contacting a conductive pin and the like with an electrode on which the above-described EL element is formed as a light emitting element.
Incidentally, it is unfavorable that there is increased possibility to cause defects of the light emitting elements, for example, by deteriorating the above-described electrode when an operation of contacting the conductive pin and the like with the electrode on which the above-described EL element is formed as a light emitting element is executed in a process of inspecting pixels for defects as described above. Moreover, though it is considered to use a method in which a load is given to the TFT for drive in a non-contact state by putting an electrode for inspection closer to the electrode, on which the light emitting element is formed, to form a capacitor between both the electrodes, it is difficult to use the method in an actual manner because gap adjustment between both the electrodes is extremely delicate.